"Cuck" is an insult lately beloved of bigots, spreading inexorably from Reddit to the schoolyards of America. What makes it beautiful is that it exposes the speaker's interest in a relatively obscure category of porn centered on the menacing appeal of big black dicks to scared white men: that being by far the most widespread use of the term online before it ascended to the pantheon of inadvertently-revealing insults.
Cuck, then, is the ne plus ultra of insults that say more about the user than the target. GQ tackles the problematic history of the alt-right's favorite new insult.
An insult is, by nature, telling of its source: you never insult with something that you don't think is insulting. A woman would never sneer that another woman is fat if she herself would be comfortable with her body at any size, if "fatness" weren't something she feared. A man mocking the size of another man's genitals broadcasts his own belief that the length of one's penis is something to be either proud or embarrassed about.
The cultural importance of the cuckold in America is rooted in racism: in pornography, the wife of the cuckolded (almost exclusively white) husband is most commonly sleeping with African-American men, meant to provide an additional layer of humiliation if the white husband sees that man as "inferior." In the world of pornography meant to elicit humiliation as an erotic sentiment, cuckold porn takes advantage of its viewers' racist perceptions.